Kulsrud family

I am trying to research my grandfather's, Oskar Vilhelm Kulsud, family. Oskar was born in 1891 in Gjerdrum, Akershus, Norway and emigrated to the US in 1907, settling in the Austin, MN area. I know most of his siblings but run into dead ends with his parentage. Everything my mother told me has been forgotten. Any help would be most appreciated. Thanks!